1. Field of the Invention
The present invention relates to photosensitive compositions and the use of these compositions in forming solid-state imaging devices. More particularly, the photosensitive compositions of the present invention contain a hydrophilic colloid, e.g., gelatin, a dichromate photocrosslinking agent and a latex copolymer.
2. Discussion Relative to the Prior Art
One of the earliest known photosensitive compositions was a composition containing a natural colloid such as gelatin and a dichromate photocrosslinking agent. Exposure of this composition produces hardening of the gelatin in the exposed areas, while the unexposed areas can be easily washed away with water. This composition, which is commonly referred to as dichromated gelatin, has been used in a variety of processes. For example, a support coated with such a composition can be used as a lithographic plate.
One use for dichromated gelatin has been in the preparation of additive color screens. (See, generally, Friedman, History of Color Photography, American Photographic Publishing Company, 1944, chapter 13.) In a typical process for producing an additive color filter screen or color filter array, the first step is to coat a suitable support with a dichromated gelatin composition. The resulting layer is then exposed in those areas where it is desired to have the individual color filter elements, and the layer is processed to wash off unwanted areas of the gelatin layer. The resulting islands or stripes of gelatin are then dyed with a suitable dye solution to form a set of colored filter elements. This process is repeated any number of times to produce any kind of desired multicolor filter array. This process has been used to produce an additive color screen for a diffusion transfer transparency material. (See, for example, U.S. Pat. No. 3,284,208 to Land.)
Another process in which the dichromated gelatin photosensitive composition is useful is in the heat-transfer process of preparing color filter arrays. This process is described in U.S. Pat. No. 4,081,277 by Brault et al issued Mar. 28, 1978. In this process, the individual color filter elements are made by heat-transferring dye into a dye-receiving layer. The dichromated gelatin photosensitive composition is used as a photoresist. The dichromated gelatin layer is exposed to the negative of the pattern of the desired filter elements and then processed to produce window areas which correspond to the filter elements in the final array. The heat-transferable dye is then transferred through the window areas of the dichromated gelatin photoresist into the dye-receiving layer, either from dye-coated paper or a hot dye solution to form the individual filter elements.
It will be readily appreciated that, either where the dichromated gelatin photosensitive composition is to be used to form islands or stripes or where it is to be used as a negative-working photoresist, it must meet several stringent requirements. Perhaps most importantly, the dichromated gelatin photosensitive composition must have extremely high resolution. In both of the above-identified processes, the dichromated gelatin photoresist determines the resolution of the individual filter elements in the color filter array. Further, in the heat-transfer process the dichromated gelatin photosensitive composition must adhere well to the polymeric dye receiver, typically a polyester. Finally, the dichromated gelatin photosensitive composition must be reproducible.
The art suggests some ways in which the properties of simple dichromated gelatin compositions can be altered. For example, U.S. Pat. No. 2,687,958 suggests that polymeric amides of acrylic acid can be used in place of all or part of the colloid. This is said to improve uniformity of the light-sensitive layers. However, it has been found that these compositions do not adhere adequately to polyester layers.
It is known in the art to produce a dichromated gelatin photoresist which, when exposed, becomes soluble rather than insoluble. Compositions of this type include, in addition to the gelatin and dichromate sensitizer, a latex which is comprised of polymer particles which are substantially completely hydrophobic. Useful hydrophobic latices for these compositions include aqueous dispersions of poly(methyl acrylate) or poly(ethyl acrylate) or a copolymer of ethyl acrylate and acrylonitrile. Compositions of this type are disclosed in U.S. Pat. No. 2,604,388 by Staehle issued July 22, 1952. Compositions of this type, i.e., those which contain latices which are substantially hydrophobic, do not provide adequate adhesion to polyester layers.
While the dichromated gelatin photoresists of the prior art can be used in both of the processes for making color filter arrays discussed above, further improvements in the areas of resolution, adhesion and reproducibility have continued to be sought. More particularly, it would be desirable to provide a high-resolution, negative-working, dichromated gelatin photoresist which can be used directly in making color filter arrays by the "wash-off" method or which can be used in making color filter arrays using the heat-transfer process. In the latter process, it is imperative that the improved dichromated gelatin photoresist have excellent adhesion to polymeric dye-receiving layers, particularly polyester layers.